Pt surface

A catalyst may play an active role in a different sense. There are interesting temporal oscillations in the rate of the Pt-catalyzed oxidation of CO. Ertl and coworkers have related the effect to back-and-forth transitions between Pt surface structures [220] (note Fig. XVI-8). See also Ref. 221 and citations therein. More recently Ertl and co-workers have produced spiral as well as plane waves of surface reconstruction in this system [222] as well as reconstruction waves on the Pt tip of a field emission microscope as the reaction of H2 with O2 to form water occurred [223]. Theoretical simulations of these types of effects have been reviewed [224]. 

The CO oxidation occurring in automobile exhaust converters is one of the best understood catalytic reactions, taking place on Pt surfaces by dissociative chemisoriDtion of to give O atoms and chemisoriDtion of CO, which reacts with chemisorbed O to give CO, which is immediately released into the gas phase. Details are evident from STM observations focused on the reaction between adsorbed O and adsorbed CO [12]. 

It is well known that the catalytic oxidation of CO on certain Pt surfaces exhibits oscillatory behavior, within a restricted range of pressures and temperatures, which are coupled with adsorbate-induced surface phase transitions [16,17]. In fact, in their clean states the reconstructed surfaces of some crystallographic planes, e.g. Pt(lOO) and Pt(llO), are... 

The experimental investigation was performed by depositing copper films on the (100) -surface of a platinum single crystal. It was found that the reconstruction of the Pt surface was lifted upon Cu adsorption. The system was then heated to different temperatures and the formation of different ordered surface alloys was evidenced by... 

The electrical double-layer structure of a Pt/DMSO interface has been investigated using the potentiostatic pulse method.805 The value of C at E = const, as well as the potential of the diffuse layer minimum, have been found to depend on time, and this has been explained by the chemisorption of DMSO dipoles on the Pt surface, whose strength depends on time. Eg=Q has been found11 at E = -0.64 V (SCE in H2O). 

Recently, with the improvement achieved in the preparation and control of surfaces, a number of approaches have been devoted to the estimation of the pzc of pt(lll).140 197 210 211 These are summarized in Table 29 for convenience of the reader. The value recommended for pc-Pt is also reported for comparison. In three cases the pzc has been estimated indirectly and the value is strikingly close to the pzc of poly crystalline Pt. In view of the heterogeneity of Pt surfaces, this closeness is puzzling and suggests that the phenomenon used to estimate the pzc does not conform to the concept of zero charge. 

The oxidation of CO on Pt is one of the best studied catalytic systems. It proceeds via the reaction of chemisorbed CO and O. Despite its complexities, which include island formation, surface reconstruction and self-sustained oscillations, the reaction is a textbook example of a Langmuir-Hinshelwood mechanism the kinetics of which can be described qualitatively by a LHHW rate expression. This is shown in Figure 2.39 for the unpromoted Pt( 111) surface.112 For low Pco/po2 ratios the rate is first order in CO and negative order in 02, for high pco/po2 ratios the rate becomes negative order in CO and positive order in 02. Thus for low Pcc/po2 ratios the Pt(l 11) surface is covered predominantly by O, at high pco/po2 ratios the Pt surface is predominantly covered by CO. 

Consequently upon adding Li on the Pt surface kco(=ko) decreases and ko (=kA) increases. Thus from Eq. (2.29) one expects a decrease (poisoning) in the rate under CO lean conditions and an increase (promotion) in the rate under CO rich conditions. This is exactly what Figure 2.39 shows for moderate Li coverages. Note that when the Li coverage, 0P, becomes too high (>0.4) then the (1-0P) term in Eq. (2.29) dominates and Li poisons the rate under both CO lean and CO rich conditions. 

The same experimental procedure used in Fig. 4.15 is followed here. The Pt surface is initially (t < - 1 min) cleaned from Na via application of a positive potential (Uwr=0.2 V) using the reverse of reaction (4.23). The potentiostat is then disconnected (1=0, t=-lmin) andUWR relaxes to 0 V, i.e. to the value imposed by the gaseous composition and corresponding surface coverages of NO and H. Similar to the steady-state results depicted in Fig. 4.18 this decrease in catalyst potential from 0.2 to 0 V causes a sixfold enhancement in the rate, rN2, of N2 production and a 50% increase in the rate of N20 production. Then at t=0 the galvanostat is used to impose a constant current I=-20 pA Na+ is now pumped to the Pt catalyst surface at a... 

It is important to observe that the electrochemically promoted Pt surface (Uwr O V) gives SN2 selectivity values above 70% vs 35% on the unpromoted surface (Uwr>0 V). The Pt surface is thus made as selective as a Rh surface would be under similar conditions. The ability of electrochemical promotion to alter the product selectivity of catalyst surfaces is one of its most attractive features for practical applications. 

Figure 4.26. Transient response of the rate of CO2 formation and of the catalyst potential during NO reduction by CO on Pt/p"-Al2C>396 upon imposition of fixed current (galvanostatic operation) showing the corresponding (Eq. 4.24) Na coverage on the Pt surface and the maximum measured (Eq. 4.34) promotion index PINa value. T=348°C, inlet composition Pno = Pco = 0.75 kPa. Reprinted with permission from Academic Press.

Both the TPD spectra (Fig. 5.2b) and the cyclic voltammograms (Fig. 5.2c) show clearly the creation of two distrinct oxygen adsorption states on the Pt surface (vs. only one state formed upon gas phase 02 adsorption, Fig. 5.2b, t=0). 

Figure 5.22 reveals the ability of solid state electrochemistry to create new types of adsorption on metal catalyst electrodes. Here oxygen has been supplied not from the gas phase but electrochemically, as 02 via current application for a time, denoted tj, of 1=15 pA at 673 K, i.e. at the same temperature used for gaseous O2 adsorption (Fig. 5.21). Figure 5.23 shows the effect of mixed gaseous-electrochemical adsorption. The Pt surface has been initially exposed to po2 =4x1 O 6 Torr for 1800 s (7.2 kL) followed by electrochemical O2 supply (1=15 pA) for various time periods ti shown on the figure, in order to simulate NEMCA conditions. 

Figure 5.39. Characterization of the spillover species by photoelectron spectra of the Ols region taken from a 0.02 pm2 spot on the Pt surface (a) The residual O Is spectrum after the cleaning cycles (b) The Ols spectrum measured in 02 atmosphere (pO2=lxI0 6 mbar) (c) The Ols spectrum obtained during electrochemical pumping in vacuum with UWr = 1.1 V. R1 and R2 are the components which are formed by adsorption from the gas phase and by electrochemical pumping. The fitting components of the residual oxygen are shown with dashed lines. Photon energy = 643.2 eV, T 350-400°C.67 Reprinted with permission from Elsevier Science.

The technique of SPEM allows one to obtain XPS spectra from extremely small (-0.02 pm2) surface areas and thus one can study O Is spectra obtained from small (-0.02 pm2) spots on the Pt surface.67... 

Figure 5.39a shows the residual O Is spectrum obtained in ultra-high-vacuum after repeated cleaning cycle at 350-400°C. It is clear that there is a significant amount of residual O on the Pt surface which cannot be removed with conventional cleaning procedures. This by itself suffices to prove the presence of the omnipresent backspillover-formed effective double layer on the vacuum exposed Pt surface. 

Figure 5.39 b shows the O Is spectmm measured in an 02 atmosphere (pO2=10 6 mbar). It is clear that the oxygen present on the Pt surface increases and the broad Ols spectmm peak shifts to 530.5 eV. 

The backspillover O species on the Pt surface have an O Is binding energy 1.1 eV lower than on the same surface under open-circuit conditions. The Pt catalyst-electrode is surrounded by isoenergetic oxygen species both at the Pt/YSZ and at the Pt/vacuum interfaces.67... 

In summary in situ XPS with metal/YSZ catalyst-electrodes has positively confirmed the O backspillover mechanism as the cause of NEMCA and has provided very interesting information about the strongly anionic state of the backspillover oxygen species. On the basis of the energetic indistinguishability of the backspillover Cf on the Pt surface and O2 in the YSZ revealed by XPS, it appears almost certain that Cf is. Nevertheless and in anticipation of... 

A lucid example is shown in Fig. 5.42. The Na Is spectmm corresponding to UWr=600 mV corresponds to an electrochemically cleaned Pt surface, thus all the Na Is signal originates from Na+ in the (U-AI2O3, visible through microcracks of the Pt film. Upon decreasing UWr one clearly observes ... 

The creation of a new peak, at 1072.8 eV, corresponding to Na present on the Pt surface. Lambert and coworkers5 60 have also shown that the same Na species forms on the catalyst surface via gas phase Na adsorption and that this species can then be pumped electrochemically into the p"-Al203 solid electrolyte via positive UWr application. 

It is worth noting that each Na atom appears to perturb the electron density of the Pt(lll) surface over large ( 12) atomic distances. This can explain nicely the observed long-range promotional effect of Na on Pt surfaces. It is strongly reminiscent of the IR spectroscopic work of Yates and coworkers who showed that a single adsorbed alkali atom can affect the IR spectra of up to 27 coadsorbed CO molecules.80... 

It should be clear that, as well known from the surface science literature (Chapter 2) and from the XPS studies of Lambert and coworkers with Pt/(3"-A1203 (section 5.8), the Na adatoms on the Pt surface have a strong cationic character, Nas+-5+, where 5+ is coverage dependent but can reach values up to unity. This is particularly true in presence of other coadsorbates, such as O, H20, C02 or NO, leading to formation of surface sodium oxides, hydroxides, carbonates or nitrates, which may form ordered adlattices as discussed in that section. What is important to remember is that the work function change induced by such adlayers is, regardless of the exact nature of the counter ion, dominated by the large ( 5D) dipole moment of the, predominantly cationic, Na adatom. 

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